This invention relates to a nuclear imaging device, e.g., a gamma-camera of the type disclosed in Anger U.S. Pat. No. 3,011,057, and more particularly, to a nuclear imaging device designed to reduce spatial non-linearity.
The head of a typical Anger-type gamma camera comprises a scintillation crystal responsive to radiation stimuli for producing light events at spatial locations corresponding to the locations at which the stimuli interact with the crystal, and an array of photomultiplier tubes positioned with their photocathodes facing the crystal and arranged in a hexagonal pattern. Interposed between the array of photomultiplier tubes and the crystal is a transparent light coupling means comprising a glass sheet covering the crystal and a light pipe of plastic material. Typically, nineteen photomultiplier tubes are arranged in a 3-4-5-4-3 hexagonal pattern. The photomultiplier tubes have overlapping fields of view with the result that a light event occurring anywhere in the crystal is detected by a plurality of the photomultipliers. Those photomultipliers close to the light event will respond by producing an output signal of greater amplitude than the amplitude of the output signals produced by photomultipliers more remote from a light event. The coordinates of a light event can be computed by processing the output signals produced as a consequence of the light event in accordance with different procedures that are well known in the art. For example, the processing can be in the manner disclosed in U.S. Pat. No. 3,011,057 or in the manner disclosed in copending application Ser. No. 503,767 filed Sept. 6, 1974 now U.S. Pat. No. 4,060,730 issued Nov. 29, 1977.
A basic problem with a gamma camera of the type described above arises from the spatial non-linearity in converting light events into position coordinate electrical signals. This problem is manifested in the flood image obtained by uniformly illuminating the crystal. Such image should be uniform but, in actuality, the image contains so-called "hot spots" wherein the image is brighter within the spots as compared to their surrounding regions. These spots are located symmetrically about the optical axes of the photomultiplier tubes. Moreover, the phenomenon is most pronounced in the central region of the crystal.
Because of the spatial non-linearity of gamma cameras, the computed coordinates of a light event occurring in the central portion of the crystal and within the volumetric projection of the photocathode of a photomultiplier will be displaced from the actual location of the light event in a direction toward the optical axis of the photomultiplier. The general solution to this problem is to alter the light distribution from the crystal to the photomultipliers by modifications involving one or more of the crystal, light pipe or photomultiplier. A specific approach to the solution to this problem by modifying the light pipe is disclosed in U.S. Pat. No. 3,859,531 wherein optically coatedtransparent cones of the same material as the light pipe are inserted in conical recesses in the scintillator side of the light pipe within the volumetric projection of the photocathode of the photomultiplier tubes in the central region of the crystal. The coating on the cones is a thin layer of transparent material with an index of refraction considerably lower than that of the light pipe. The cones alter the distribution of light from light events occurring at various positions under each of the photomultiplier tubes in the central region such that the resultant response of the camera system shows considerably improved spatial linearity.
In order to practice this invention, it is necessary to accurately fashion the transparent cones and the conical recesses for receiving the cones in order for a good fit to be achieved. If the fit between a conical cone and the conical recess is not uniform throughout the region of contact between the cone and recess, the effect of the cone on light incident thereon will be angularly dependent thus introducing a further type of non-linearity.
It is therefore an object of the present invention to provide a new and improved nuclear imaging device which overcomes or substantiall reduces the deficiencies in the prior art devices.